Field
Embodiments of the invention relate to the field of optical coherence tomography.
Background
Optical Coherence Tomography (OCT) is a technique for the generation of medical images that can provide axial information at a high resolution using a broadband light source and an interferometric detection system. It has found a wide range of uses, from cardiology to ophthalmology and gynecology, and for in-vitro sectional studies of biological materials.
Axial information is obtained in OCT through interferometric methods. One approach to generate images (2D) and volume representations (3D) of the histology of tissue is to move the beam laterally over the area of interest. This movement has been traditionally done by means of mechanical displacement of some optical element within the system, such as the waveguide in the case of fiber-based systems. Alternatively, the sample can be moved underneath a stationary beam. The most common solution utilizes a moving mirror in the beam path in the sample arm of the interferometer. Although this method is effective, it has drawbacks, especially in terms of reliability, manufacturing cost, maintenance cost, complexity of adjustment, final system size, etc. The use of MOEMS technology (Micro-opto-electromechanical systems) has been proposed and demonstrated for situations in which conventional mirrors are not acceptable, such as in catheters or laparoscopic instruments. However, these devices suffer from many of the same problems as their macroscopic versions and they pose their own challenges in terms of encapsulation, sterilization, etc.
One approach for providing a lateral scan over a sample is to use multiple beams. An example of this was proposed in WIPO Patent Application Publication WO 2010/134624. Several complete interferometers working in parallel are described that only share the light source. As such, the sample arm of every interferometer consists of a single optical path, leading to a structurally complicated system.
High-speed Optical Coherence Tomography (OCT) imaging is important for 3D scans of large tissue volumes, for the evaluation of fast dynamics in the sample and in indications prone to motion artifacts because of mechanical instability or body movements. Meeting this goal may require a significant increase in actual acquisition speed beyond the line speed of a single axial scanner (which is fixed by the physical properties of the external cavity or delay line) to obtain a sufficiently high sampling rate. Also, it may require improving the signal-to-noise (SNR) of the system to ensure good image quality in spite of high speeds. After optimization of system optics and electronics, and given usage of Swept Source OCT (SS-OCT) or Time Domain OCT (TD-OCT) implementations, this translates into increasing the maximum tolerable optical radiation limits through an extension of its optical Etendue.
Full-field OCT systems may meet these goals a priori because of their construction. However, they may suffer from cross-talk between adjacent channels and image quality problems. Full-field OCT (FF-OCT) systems also require 2D imaging sensors, which may limit them practically to wavelengths where such sensors are affordable and have sufficient resolutions (currently only <1 μm). Such sensors also limit image acquisition speed to the frame rate of the imager. Line-scan OCT limits the parallel acquisition to a single line and uses a scanning element to gain additional directions. Although cross-talk is better than in FF-OCT, image quality is still significantly worse than in standard OCT.
A solution described in WIPO Patent Application Publication WO 2014/089504 uses a spatially expanded source that is conformed into a plurality of separate beams by means of a mechanically actuated mask. The beams are then scanned over the surface of the tissue to be analyzed in order to produce the images. As long as the separation of these beams is large (the sampling is sparse), cross-talk can be effectively reduced. The problems with this approach are the loss in optical throughput when the source is masked, the need for a 2D imager (especially at longer wavelengths), and the trade-off between dense sampling and cross-talk.
Another approach described in Japanese Patent Application Publication JP 2010276462 uses an OCT system with multiple interferometers for a Time Domain configuration having 1 or 2 superluminescent diodes (SLEDS), but does not avoid multiple scattering cross-talk.